1. Technical Field
The present invention relates generally to a control apparatus for use with an electric power conversion system which is equipped with a dc/ac converter connected to an electric rotating machine and a capacitor joined to input terminals of the dc/ac converter, and more particularly to such a control apparatus designed to control an operation of the dc/ac converter so as to decrease the voltage charged in the capacitor below a given level.
2. Background Art
Japanese Patent First Publication No. 9-70196 discloses a control apparatus designed to control energization of a three-phase permanent-magnet electric motor so as to set a command current in a q-axis direction to zero (0) and an absolute value of a command current in a d-axis direction to more than zero (0), thereby discharging a capacitor connected to an inverter. This manner of control is based on the fact that the torque T to be produced by the motor is expressed by T=p{ψ+(Ld−Lq)id·iq} where id is a d-axis current, iq is a q-axis current, Ld is a d-axis inductance, ψ is a coefficient of flux linkage of an armature of the motor, and p is the number of pairs of poles of the motor. Specifically, the phase of current to energize the motor (which will also be referred to below as a motor-energizing current) is oriented to the d-axis in order to avoid the generation of torque upon the energization of the motor.
The control of energization of the motor to set the command current to zero (0) requires use of a measured value of an electrical angle of the motor. If the measured value has an error, the orientation of the phase of the motor-energizing current to the d-axis based on the measured value may result in generation of torque in the motor. When the motor is rotated by such torque, it will cause the phase of the motor-energizing current continues to be oriented to the d-axis, as defined in error. The motor, thus, continues to produce the torque, in other words, to rotate.
Additionally, the factor that the torque does not become zero (0) is also attributed to an error in the above torque equation. Specifically, the torque equation is derived in a simplified motor model wherein the values of the d-axis inductance Ld and the q-axis inductance Lq are fixed. Typical electric motors, however, have an inductance component which arises from the space harmonics and is dependent on the electrical angle of the motor. This may cause the torque to be produced undesirably even when the phase of the motor-energizing current is oriented to the d-axis in the control of energization of the motor. Such a problem is becoming serious with an increase in use of permanent magnet motors equipped with concentrated windings in which the space harmonics will appear strongly.